share.html: Apply aria-hidden to share-box (#42)

The share box is not accessible to screen readers since it does not
contain text. Thus, exclude it from screen-readers via aria-hidden.

Signed-off-by: Stephan Kulla <git.mail@kulla.me>

layouts/partials/share.html

@@ -1,5 +1,5 @@
 {{ if .Site.Params.sharing }}
-<div class="share-box">
+<div class="share-box" aria-hidden="true">
   <ul class="share">
     <li>
       <a class="facebook"